1. Field of the Invention
The present invention relates generally to the detection of high or low levels of fluid and more particularly to a level detection system which may be continuously validated to ensure proper operation.
2. Background of the Invention
It is often necessary or desirable to monitor the level of fluids within various containers and process vessels. For example, in connection with various chemical processes, fluids are processed within vessels and it is important for those managing the process to know when there is either a low level fluid condition or a high level fluid condition within the process level at any time during processing. In most cases, these conditions can not be detected merely by looking at the vessel since the vessel itself is typically non-transparent and, as a result, fluid levels would be unknown without one or more sensors being contained within the process vessel itself.
If fluid levels are too low, more fluid must typically be introduced into the vessel to ensure that the chemical process within the vessel continues without problems or to ensure that the vessel does not run dry creating the potential for equipment problems. This can be done manually by, for example, a technician opening a valve for some period of time to introduce additional fluid. Alternatively, or in addition, additional fluid may be added automatically by a control system which may, for example automatically open a valve to introduce additional fluid upon detection of a low-level condition. In the case of a high level condition, fluid may be drained though the use of a valve typically located towards the bottom of the vessel either automatically or manually as described above with respect to low level conditions.
In current practice, conventional level switches are used in connection with Safety Instrumented Systems (SIS) within the process industries. These switches typically activate at high or low levels in vessels and are used to signal automated control systems to prevent vessels from overfilling or running dry. However, with these switches, it is not possible to easily ensure that the switch will work and detect the out of range level at the critical time.
In most cases, with existing switches, it is necessary to manually test the switches to confirm that the switch will operate on demand and that there are no covert failures of the switch which would result in a failure to operate in a high or low fluid level situation. Unfortunately, such manual testing is a very time intensive task. For example, manual testing may be required to be performed at a pre-specified scheduled interval such as every six months. In addition, manual testing can often result in undesirable situations such as a case where the technician doing the test causes a false trip of the unit during testing. Even worse, manual testing may result in a situation where the technician unintentionally leaves the switch in a “bypassed” condition after a test such that during operation, out of range fluid levels will not be properly detected and/or remedied.
With current level detection switches functioning in an “on” or “off” status, as opposed to over an analog scale, it is not possible to employ other sensing elements within the vessel to use to comparison test against the switch. As a result, validation through the use of other sensors, even if they are analog, is not a workable solution. Manual testing, therefore, is the primary method of switch validation in most cases.
The on-off type level switches currently in use are typically characterized as having a “ball-cock” type float action. The rising or falling liquid level causes the float to move up or down which, in turn, may, at prescribed liquid levels, actuate an on-off electrical relay or switch. In most cases the relays or switches currently in use have a short range between upper and lower vessel taps. Usually the range is limited to 18 inches or less. These types of switches are subject to failures which are generally undetectable. For example, the failure modes may include failing to indicate a low level when the level is lower than the prescribed low level limit or failing to indicate a high level when the level is higher than the prescribed high level limit. In order to attempt to minimize and discover failures of this sort, manual testing is typically periodically performed.